1. Field of the Invention
The present invention relates to an apparatus of high-voltage (over 6 volts) transistors and diodes on the same substrate and made in the same process as ordinary 5-volt CMOS (Complementary Metal Oxide Semiconductor) circuits and to a process for making the apparatus. More particularly, but not by way of limitation, the present invention allows logic circuitry to be fabricated in existing CMOS processes, while allowing interfacing to high-voltage circuits on the same silicon substrate.
2. Discussion
High-voltage capability has important applications in consumer electronics, automotive and other markets where many existing devices such as motors, transducers and actuators operate on high voltages. The present invention allows the logic circuitry to operate on power-saving low voltage while allowing the entire device to be fabricated on a single silicon chip in an otherwise conventional CMOS process. For the purpose of this discussion, low voltage is a voltage of 6 volts and less, while high voltage is a voltage of over 6 volts.
Conventional CMOS logic integrated circuits (hereinafter CMOS chips) operate between 2.4 and 5 volts. The range of power supply voltages at which the chip can operate is known as the operating voltage range. Typically a given chip can simply be powered with any power supply selected between 1.2 and 5 volts and the logic will operate with a logic low of 0 volts and logic high of the selected power supply voltage. Typically for battery-operated devices and other devices where a minimum of power consumption is desirable, the lowest possible voltage is used. Although the use of this low voltage minimizes the power consumed, existing physical devices such as motors, transducers, and actuators operate advantageously at higher voltages. Moreover, the trend is toward CMOS chips that operate at even lower voltages while these other components stay at their fixed voltages, and may even operate more efficiently at higher voltages.
The areas in which high-voltage CMOS chips will be useful are numerous. Applications in LCD active matrix technology, linear amplifiers, voltage translators, switching regulators, computer interfaces including the ubiquitous RS-232 interface and in computer disk drive read/write circuits are in need of this innovation. Numerous other applications will suggest themselves when the device becomes commercially available and thereby well-known. In short, any application which involves operating voltages outside of the presently limited range will benefit from the device. Moreover, as the geometry of the CMOS chips continues to shrink in size, the operating voltage range will be in even lower voltages which will greatly increase the need for the present invention.
As an example, an automobile may have a microprocessor controller that drives a 12-volt motor. For the purpose of this example, assume the motor does a particular physical task and therefore requires the same power output of 120 Watts, regardless of supply voltage. If the motor draws 10 Amperes of current through connections that through age and neglect have a resistance of 0.1 ohms, the power consumed in these connections is 10 Watts. On a 24-volt system, a motor of the same locomotive power would draw only 5 Amperes, dissipating only 2.5 Watts as heat. Similarly on a 6-volt system, the power dissipated as heat would be 40 Watts, a substantial fraction of the motor's power of 120 Watts. The ratio of power wasted to power consumed for the 6, 12, and 24-volt systems is respectively, 33%, 8%, and 2%. This illustrates how a widely used system, the electrical system of automobiles, is unlikely to go to any lower voltage because of increasing inefficiency in the system as the voltage is decreased.
CMOS chips, on the other hand, have become more efficient at lower voltages. The 4000B series CMOS operates between 5 and 15 volts and has been widely used for over twenty years. However, it accomplishes its high-voltage range by using much larger transistors than are used in current CMOS submicron geometries. As device size was reduced and new CMOS series introduced, the operating voltage range dropped until today, as an example, the 4-bit National Semiconductor COP424C CMOS microcontroller draws 3.5 milliwatts when operated at 5 volts, but only 0.29 milliwatts at 2.4 volts. Unfortunately for the car manufacturers, the upper limit of the supply voltage for this microcontroller is only 6 volts. Moreover, as modern fabrication techniques reduced device size, the typical maximum operating voltage has decreased. Using a 6-volt controller on a 12-volt system presently requires surrounding the chip with various external interface transistors at additional cost.
The additional transistors, required for the purpose of interfacing to the high voltage circuitry, became necessary as the same CMOS device miniaturization that decreased voltage and power requirements also decreased the depth of the semiconductor wells thereby decreasing the punch-through and breakdown voltages, giving rise to the lowered maximum operating voltages. The punch-through voltage of a CMOS gate is the voltage applied across the source and drain which, in and of itself and without regard to the voltage on the gate, causes significant current flow through the source and drain. Because the current flows without regard for the voltage on the gate of the device, the device is no longer functional. Moreover, the power dissipated by the device in such a condition typically destroys the device. The punch-through voltage is therefore the measure of how great a voltage can be switched by a given transistor. The breakdown voltage is analogously defined as the voltage between the gate and drain or between the gate and source which causes significant current flow through the gate. Insomuch as a CMOS transistor has an insulated gate, this too represents a breakdown of the transistor's function.
In a diode, which has only one junction, only the breakdown voltage is defined. The breakdown voltage is the voltage which reverse-biases the diode and causes significant current to flow through the diode. Unless the current is limited by external means, catastrophic overheating and subsequent destruction of the diode occurs. Because the diode has only one junction, punch-through voltage is not defined for the diode. Consequently, throughout this application, where the term punch-through occurs, the term is understood as applying only to transistors and its application to diodes is a nullity.
The punch-through and breakdown voltages in an integrated circuit are functions of the physical three-dimensional geometry of the circuit. If the resultant electric field within a device exceeds certain bounds, catastrophic electrical breakdown occurs based on the electric field and the device geometry. The geometry is controlled by the extent to and rate at which various dopants diffuse into a given substrate. Such dopants include boron, phosphorus, and arsenic.
The relative diffusion rates of the various dopants are important factors in determining the final device geometry. Different dopants diffuse at different rates and because diffusability is a steep function of temperature, the depth of diffusion is not a linear function of the time for which and temperature to which the wafer is heated. Because of this nonlinearity, heating the wafer twice for the same cumulative time does not produce the same depth of diffusion as heating the wafer once. Therefore, although the various thermal cycle times are stated and are in fact done to accomplish a particular task, the effect on the diffusion of the dopant is also a key issue in the ultimate placement of the doped regions. Consequently, final device geometry depends to a large extent on the overall sequence of events comprising the manufacture of the chip.
A need exists for, and the present invention is directed toward, a process that overcomes these shortcomings of the current CMOS apparatus of the conventional low voltage CMOS process and allows high-voltage devices to be fabricated on existing low-voltage submicron CMOS production lines.